1. Field of the Invention
The present invention relates to fuel cells, and especially to hydrogen-oxygen fuel cells, and more specifically relates to the control of the humidification of such cells.
2. Discussion of the Related Art
As illustrated in FIG. 1, a hydrogen-oxygen fuel cell comprises a layer or sheet of an electrolyte 1 sandwiched between two catalyst layers or sheets 3 and 4 coated with conductive layers 6 and 7 intended for the contacting. The upper surface of the cell is in contact with oxygen, for example, the ambient air, and the lower surface of the cell is in contact with hydrogen.
Under such conditions, when the cell is connected to a load 8, a positive voltage appears on the upper surface side or cathode and a negative voltage appears on the lower surface side or anode, and a current flows through the load. On the anode side, the catalyst transforms gaseous hydrogen molecules into two protons and two electrons, the protons travel from the anode catalyst layer, through the electrolyte layer, to the cathode catalyst layer where reaction 2H++½02+2e−→H2O takes place, with both electrons flowing through the load.
Currently, electrolyte 1 is Nafion and catalyst 3, 7, is a carbon platinum mixture, for example comprising a few percents of platinum. The catalyst also preferably contains a given amount of Nafion, for example, from 20 to 40%.
Conductors 6 and 7 for example are very thin gold layers, to be both conductive and permeable to hydrogen or oxygen. Conductors 6 and 7 may also be formed of gold grids.
FIG. 2 shows an embodiment of a fuel cell using microelectronics techniques. This cell is formed on a silicon wafer 10 that may be coated with a first thin insulating layer 11 and with a second thicker insulating layer 12. An opening is formed in a portion of insulating layer 12. In this opening are successively deposited a catalyst layer 3, an electrolyte 1, and a second catalyst layer 4 (the thicknesses of insulating layer 12 and of layers 3, 1, and 4 may be such that at least some of layers 3, 1, and 4 extend widely beyond the opening). A lower anode electrode 6 enables to take a contact on lower catalyst layer 3. An upper cathode electrode 7 enables to take a contact on upper catalyst layer 4. Electrodes 6 and 7 are provided with openings, and channels 13 are formed in silicon wafer 10 opposite to openings in the lower surface metallization. Further, an enclosure 15 has been shown on the lower surface side of the cell, this enclosure defining a chamber used as a hydrogen buffer tank which is connected to a hydrogen source or to a hydrogen generation source.
This is an embodiment only. Various types of fuel cells that may be formed as illustrated in FIG. 2 are known in the art. For example, the silicon wafer portion which supports the actual fuel cell is preferably thinned down. This thinned-down portion of wafer 10 is bored with channels 13 letting through hydrogen. It should be understood that, generally, all wafer surfaces are coated with an insulator formed at least of native silicon oxide.
Catalyst layers 3 and 4 are formed by any means, for example, by inkjet deposition. Nafion layer 1 is for example spun on. In such a fuel cell, the power likely to be provided is especially proportional to the surface area taken up by the cell in the silicon wafer plane. Currently, the useful surface area of a fuel cell of the type described in relation with FIG. 2 ranges between 1 and 3 cm2.
Such hydrogen-oxygen fuel cells are particularly appropriate for an installation in portable devices such as portable phones or computers. Their use however poses a problem since, as seen previously, the reactions associated with the cell operation imply the generation of water and the cells operate best when the water content in the electrolyte is within a limited range. In prior art, different means have been used to ensure the cell humidification, especially if said cell dries out during its operation, for example because it is placed in a very dry atmosphere, or because the very operating conditions are not favorable to the provision of water (high voltage and low current). Prior art humidification means are generally complex and for example imply the addition of water vapor to the gases (air or oxygen and hydrogen) supplying the cell. Further, the measurement of the electrolyte humidity ratio generally requiring interrupting the normal operation of the cell.